An exposure device in which individual LED arrays are energized, for example, symmetrically about an imaginary line that extends parallel to the array for variable time for each image has been disclosed in U.S. Pat. No. 4,712,116. LED exposure devices are especially useful in printers, for example, U.S. Pat. No. 4,455,562.
Generally in these devices, such as in the U.S. Pat. No. 4,455,562, the absolute light intensity of each LED on the photosensitive layer is determined, and an absolute correction number is derived from this and stored in a memory. The absolute correction number is used to energize each individual LED per image line for a time which is inversely proportional to the light intensity of said LED. In this way the LED's are energized with pulse time modulation and each LED is supposed to deliver the same amount of light to the photosensitive layer per image point.
When an LED is used, however, aging occurs. Typically, the emitted quantity of light decreases as a function of the period of use. This aging will not be equal for all the LED's because the period of use of each individual LED depends on the information content of the images under formation and also because LED's which emit less light per image line have to be energized longer than the LED's which emit more light. Thus, where automatic compensation means such as U.S. Pat. No. 4,839,672 are not incorporated into the device the absolute correction numbers must be adapted after many hours of use. In known devices such as United Kingdom Application above, the absolute light intensity of the photosensitive layer has to be continually redetermined. This means that a service engineer is required to carry out an absolute light measurement for each LED on the photosensitive layer at specific intervals or that the array of LED's will have to be removed from the printer to determine the absolute correction numbers in a test rig.
It is also known from U.S. Pat. No. 4,588,883 to provide an array of LED's on a substrate with a detector for measuring the light intensity of each LED, but this detector has the same length as the array of LED's and the absolute light intensity of each LED is always measured by means of this detector. A disadvantage of this is that it is difficult to ensure equality due to the considerable length of the detector. Local soiling causes considerable inequalities. Also, each LED is contained in a control circuit, the current through the LED being adjusted so that the absolute light intensity of the LED remains constant. Controlling the current through an LED so that the light intensity remains constant is difficult at high switching speeds because the switching-on time for the LED varies so that the individual image points per image line are not imaged on one and the same line.
Accordingly, it is an object of the invention to provide a means for determining the absolute correction numbers without the need to remove the LED array from the device. It is a further objective to provide an accurate means for compensating for the aging of an LED in an array of LED's used in printing devices.